luke - i'm 99% sure that Vostok is right. If you have a look at the racks that are extended from the MTT, then at the racks on the carrier's, you will see they are the same.

A real life example of size difference would be the M-939 Cargo Truck and the M-2 Infantry Fighting Vehicle. Although both are around the same size, the M-939 can carry a LOT more people than the M-2, because a lot of space in the M-2 is taken up by electronics, weapons, ammunition, armour etc.

That would be the same thing with the size of the MTT - because of its weapons, armour, larger engine etc, it would have to be bigger to enable it to carry the droids into combat.